Exploring the Complex Heliotail Boundary by an Extended Level Set Approach

Abstract There is an ongoing debate regarding the shape of the heliotail. Studies have shown that the heliotail may be “comet-like,” extending for thousands of au. Some previous works defined the heliopause using magnetohydrodynamic variables, which only serve as an approximation to capture the heliopause in all directions. Here we use a level set method in our heliospheric model to constrain the location of the heliopause. The level set function, f HP, is defined by boundary conditions, having a value of +1 at the inner boundary and −1 at the outer boundary. We find that the f HP = 0 criterion, the standard heliopause definition used by other models to determine where solar wind and interstellar plasma meet, is impacted in the heliotail, where there is a broad mixing region of solar wind and interstellar medium plasma, by the nature of turbulent flows in the heliotail. We explore a different isosurface, defined by f HP = 0.99, where the interface is composed of 99.5% solar wind and 0.50% interstellar medium. This determines the limits of the heliosphere by the region of solar wind flow that is minimally influenced by the interstellar flow. This region also contains magnetic field lines with at least one end anchored to the Sun. We obtain a split heliotail structure using the f HP = 0.99 criterion.